Motor vehicles, in particular motor cars, are now commonly provided with air bag restraints to prevent the vehicle passengers or driver being injured during collisions. Some vehicles are provided with air bags which in their stand-by condition are housed within a vehicle seat back or base.
On conventional seats when the air bag is activated and inflated, it escapes through the seat cover by bursting through the sewn seams where portions of the cover are sewn together. Some seat covers are provided with air bag guides which direct the inflating air bag towards the seam through which it is intended to burst.
Three dimensional seat covers are completely homogeneous and, therefore, in order to provide a knitted cover which splits or bursts in a predictable manner, the cover is knitted with a coursewise line of weakness through which the air bag can burst. This seat cover is described in the applicants' U.S. Pat. No. 5,682,771, the contents of which are hereby incorporated into the present invention.
If the seat back and base comprise bolsters, it is necessary to provide anchorage devices on the underside of the seat covers to shape the cover to the upper surface of the bolster. The anchorage device is typically an open-ended tubular portion of knitted fabric which is formed integrally by knitting, as is described in U.S. Pat. No. 5,308,141 and U.S. Pat. No. 5,326,150.
The air bag burst line may be formed close to the anchorage device so that the cover is held against outward movement as the air bag inflates.